1. Field of the Invention
The present invention relates to a sensor element and a method for detecting a measuring gas.
2. Description of the Related Art
A large number of sensor elements and methods for detecting at least one property of a measuring gas in a measuring gas space are known from the related art. In principle, this may involve arbitrary physical and/or chemical properties of the measuring gas, whereby one or multiple properties may be detected. The present invention is described below, in particular, with reference to a qualitative and/or quantitative detection of a gas component of the measuring gas, in particular, with reference to a detection of an oxygen content in the measuring gas. The oxygen content may, for example, be detected in the form of a partial pressure and/or in the form of a percentage. Alternatively or in addition, other properties of the measuring gas are also detectable, however.
For example, such sensor elements may be designed as so-called lambda sensors, as are known for example, from Konrad Reif (publisher): Sensoren im Kraftfahrzeug [Sensors in the motor vehicle], 1st edition, 2010, pp 160-165. With broadband lambda sensors, in particular with planar broadband lambda sensors, it is possible, for example, to determine the oxygen concentration in the exhaust gas in a large area, and thereby deduce the air-fuel ratio in the combustion chamber. The air ratio λ describes this air-fuel ratio.
Ceramic sensor elements, in particular, are known from the related art, which are based on the use of electrolytic properties of certain solid bodies, i.e., on ion-conductive properties of these solid bodies. These solid bodies may be, in particular, ceramic solid electrolytes such as, for example, zirconium dioxide (ZrO2), in particular, yttrium-stabilized zirconium dioxide (YSZ) and/or scandium-doped zirconium dioxide (ScSZ), which may contain small additional amounts of aluminum oxide (Al2O3) and/or silicon oxide (SiO2).
Such sensors are subject to increasing functional demands. In particular, a rapid operational readiness of lambda sensors after an engine start plays a significant role. This readiness is influenced essentially by two aspects. The first aspect relates to a rapid heating of the lambda sensor to its operating temperature above 600° C., which may be achieved by a corresponding design of a heating element or by a reduction of the area to be heated. The second aspect relates to the robustness against thermal shock as a result of water hammer during an operation. The aforementioned thermal shock is due to the fact that for a certain period of time after the engine start, the temperature in the exhaust pipe lies below the dew point for water, so that water vapor formed during fuel combustion is able to condense in the exhaust pipe. This leads to the formation of water droplets in the exhaust pipe. Due to the impact of water droplets, the heated ceramic of the lambda sensor may be damaged or even destroyed as a result of thermal stresses or fractures in the sensor ceramic. For this reason, lambda sensors have been developed which have a porous ceramic protective layer on their surface, also referred to as a thermal shock protection layer. This protective layer ensures that water droplets impacting the lambda sensor are distributed over a wide area, thereby reducing the locally occurring temperature gradients in the solid body electrolyte or sensor ceramic. Thus, in the heated state, these lambda sensors tolerate a certain droplet size of condensed water without being damaged. The protective layer is normally applied to the sensor element in an additional method step. Various materials such as, for example, aluminum oxide or spinel (MgAl2O4) and coating techniques such as, for example, spray processes or immersion processes are used for this purpose.
In spite of the numerous advantages of the methods for manufacturing sensor elements for lambda sensors known from the related art, there is nevertheless potential for improvement.